This invention relates to a method of scanning and imaging objects that are larger than a detector array, and more particularly to a translate rotate scanning motion for digital radiography (DR) and computerized tomography (CT) imaging.
Rapid inspection of large industrial parts is becoming increasingly critical as manufacturing moves from assembly of small parts to utilization of large castings. In order to meet typical spatial resolution requirements, detectors with many thousands of individual elements are becoming necessary. In addition, as high resolution scanning capabilities and requirements grow, inspection of even small parts can require thousands of detector elements. Fabrication requirements, however, limit the number of individual elements that can be assembled into a single detector. The problem presented is to create a scanning configuration that can synthesize large arrays of detector elements from smaller arrays in a rapid, efficient manner, and to devise image reconstruction algorithms which make use of these data in an optimum fashion.
CT imagers have evolved through several generations of devices as image size and throughput requirements have grown. First generation scanners, FIG. 1a, utilize a source 10, which may have a collimator to shape the beam, and a single detector element 11 to acquire data for a CT image. The part 12 is translated laterally past the source and detector at a first view angle, rotated and scanned laterally at the second view angle, and so on until many view angles over 180 degrees have been covered and a parallel beam data set has been acquired point by point in series. Second generation scanners, FIG. 1b, use a number of discrete detectors aimed at the source at different angles over a given range, .theta..sub.R, to allow acquisition of data a number of view angles simultaneously. Multiple element detector 13 has wide element spacing and the four view angles cover .theta..sub.R. Part 12 is translated and scanned past the source and detectors to collect all the data for the angles represented. It is then rotated by angle .theta..sub.R, and scanned laterally again for the new range of view angles. The process is repeated N times until N.theta..sub.R is greater than or equal to 180 degrees.
Third generation CT scanners, FIG. 1c, speed up the process significantly by using a linear array of detectors to acquire all the data at a fixed view angle simultaneously. Multiple element detector 14 has narrow element spacing and the size of part 12 is smaller than the field of view of the detector. Only rotational scanning of the part 12 through 360 degrees is required to acquire the data for the various view angles. However, the data is acquired in fan beam configuration, and the detector array must be wide enough to span the part.